HID: magicmouse: properly account for scroll movement in state

Before this change, sequential scroll events would take a variable
amount of movement due to incorrect accounting. This change ensures all
scroll movements require a deterministic touch movement for an action to
occur.
Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Michael Poole <mdpoole@troilus.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-magicmouse.c

@@ -189,7 +189,8 @@ static void magicmouse_emit_touch(struct magicmouse_sc *msc, int raw_id, u8 *tda
 		case TOUCH_STATE_DRAG:
 			step = step / accel_profile[msc->scroll_accel];
 			if (step != 0) {
-				msc->touches[id].scroll_y = y;
+				msc->touches[id].scroll_y -=
+					step * accel_profile[msc->scroll_accel];
 				msc->scroll_jiffies = now;
 				input_report_rel(input, REL_WHEEL, step);
 			}